Magnesium alloy sheet

ABSTRACT

A magnesium alloy sheet formed of a magnesium-based alloy containing, on a mass percent basis, Al: 5.0% or more and 6.5% or less, Sr: 0.2% or more and 1.0% or less, Zn: 0.1% or more and 0.75% or less, and Mn: 0.1% or more and 0.5% or less, the remainder being magnesium and incidental impurities.

TECHNICAL FIELD

The present invention relates to a magnesium alloy sheet.

The present application claims the priority of Japanese PatentApplication No. 2017-122544, filed Jun. 22, 2017, which is incorporatedherein by reference in its entirety.

BACKGROUND ART

Patent Literature 1 discloses a magnesium alloy sheet with smallelongation anisotropy in warm working and with good warm plasticformability, which is formed of a magnesium alloy containing 9% by massAl.

CITATION LIST Patent Literature

PTL 1: WO2009/001516

SUMMARY OF INVENTION

A magnesium alloy sheet according to the present disclosure is formed ofa magnesium-based alloy containing,

on a mass percent basis,

Al: 5.0% or more and 6.5% or less,

Sr: 0.2% or more and 1.0% or less,

Zn: 0.1% or more and 0.75% or less, and

Mn: 0.1% or more and 0.5% or less, the remainder being magnesium andincidental impurities.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows images of cross sections of rolled materials of samples No.10 and No. 101 in Test Example 1 taken with a scanning electronmicroscope.

FIG. 2 is an image of a cross section of a rolled material of the sampleNo. 10 in Test Example 1 taken with a scanning electron microscope.

FIG. 3 shows images of cross sections of cast materials of the samplesNo. 10 and No. 101 in Test Example 1 taken with a scanning electronmicroscope.

FIG. 4 shows images of cross sections of the cast materials of thesamples No. 10 and No. 101 in Test Example 1 taken with an opticalmicroscope.

DESCRIPTION OF EMBODIMENTS Problems to be Solved by Present Invention

A magnesium alloy sheet with high ductility and corrosion resistance isdesired.

Magnesium alloys rich in Al, typically AZ91 alloys and magnesium alloyswith an Al content comparable to the Al content of AZ91 alloys accordingto American Society for Testing and Materials standard, have highcorrosion resistance but low ductility due to the high Al content. Amagnesium alloy sheet described in Patent Literature 1 has a highelongation in warm working at 200° C. or more and high ductility in warmworking but a low elongation of approximately 15% or less at roomtemperature, for example, at 20° C. Thus, there is a demand for amagnesium alloy sheet with a high elongation after fracture and highductility even at room temperature and with high corrosion resistance.

Accordingly, it is an object of the present disclosure to provide amagnesium alloy sheet with high ductility and corrosion resistance.

Advantageous Effects of Present Invention

A magnesium alloy sheet according to the present disclosure has highductility and corrosion resistance.

Description of Embodiments of Present Invention

First, the embodiments of the present invention are described below.

(1) A magnesium alloy sheet according to an embodiment of the presentinvention is formed of a magnesium-based alloy containing,

on a mass percent basis,

Al: 5.0% or more and 6.5% or less,

Sr: 0.2% or more and 1.0% or less,

Zn: 0.1% or more and 0.75% or less, and

Mn: 0.1% or more and 0.5% or less, the remainder being magnesium andincidental impurities.

The magnesium alloy sheet formed of the magnesium-based alloy with aparticular composition containing Al, Sr, Zn, and Mn in these particularranges has high ductility even at room temperature and high corrosionresistance, as described below. The term “room temperature”, as usedherein, refers to 20° C.±15° C.

Magnesium based alloys with a high Al content, such as AZ91 alloys, arelikely to form Mg₁₇Al₁₂, which is effective for corrosion resistance,and have high corrosion resistance. However, a high Al content tends toresult in coarsening of Mg₁₇Al₁₂, and casting defects tend to occuraround the coarse grains. These coarse grains or casting defects tend todecrease elongation after fracture and ductility. Although a low Alcontent results in a decrease in coarse Mg₁₇Al₁₂ or casting defects andhigh ductility, it results in low corrosion resistance. In contrast, themagnesium alloy sheet, which has a particular Al content lower than theAl content of AZ91 alloys and has a particular Sr content, canpreferentially form an intermetallic compound containing Sr (hereinafteralso referred to as a Sr-based compound), typically Al₄Sr, over Mg₁₇Al₁₂particularly in the production process. The preferential formation tendsto result in fine Mg₁₇Al₁₂. Furthermore, a relatively low Al content asdescribed above rarely results in coarsening of a Sr-based compound,such as Al₄Sr. Thus, coarse Mg₁₇Al₁₂ or Al₄Sr grains and casting defectsaround these coarse grains can be decreased in the magnesium alloysheet. Hence, the magnesium alloy sheet is less prone to fracture due tothese coarse grains or casting defects and has a high elongation afterfracture and high ductility even at room temperature.

In general, rolled sheets formed of known magnesium alloys often havedifferent mechanical properties (elongation after fracture, etc.)between the rolling direction and a direction perpendicular to therolling direction (hereinafter also referred to as a transversedirection), that is, anisotropy of mechanical properties. Typically, themechanical properties in the transverse direction are inferior to themechanical properties in the rolling direction. For example, a largedifference between elongation after fracture in the rolling directionand elongation after fracture in the transverse direction results in thedifficulty of uniform bending in any direction with high accuracy. Dueto such anisotropy of mechanical properties, the rolled sheet is oflimited use. In contrast, the cause of fracture can be alleviated in themagnesium alloy sheet, as described above. Thus, the magnesium alloysheet has a small difference between elongation after fracture in therolling direction and elongation after fracture in the transversedirection, that is, small anisotropy of elongation after fracture, andtherefore has high ductility also in this regard. A method for measuringelongation after fracture in the rolling direction and elongation afterfracture in the transverse direction is described in detail below inTest Example 1.

Furthermore, the magnesium alloy sheet contains Zn in a particular rangeas well as Al and Sr. Zn probably contributes to partly promotedformation of Mg₁₇Al₁₂ after the preferential formation of the Sr-basedcompound. Thus, the magnesium alloy sheet can appropriately containMg₁₇Al₁₂ and has high corrosion resistance. Furthermore, the magnesiumalloy sheet contains Mn in a particular range as well as Al, Sr, and Zn,and Fe, which has adverse effects on corrosion resistance, can exist asan Al—Mn—Fe compound. This also provides the magnesium alloy sheet withhigh corrosion resistance.

The magnesium alloy sheet has high ductility even at room temperatureand, unlike known sheets, has high corrosion resistance irrespective ofa relatively low Al content of 6.5% or less by mass. Thus, the magnesiumalloy sheet can also be utilized as a material for an application thatrequires high ductility at room temperature.

(2) In one embodiment of the magnesium alloy sheet,

the difference between elongation after fracture in the rollingdirection and elongation after fracture in a transverse direction in themagnesium alloy sheet is 2% or less.

This embodiment has small anisotropy of elongation after fracture andhigh ductility even at room temperature.

(3) In one embodiment of the magnesium alloy sheet,

the elongation after fracture is 18% or more.

This embodiment has a high elongation after fracture and high ductilityeven at room temperature. As in (2), the magnesium alloy sheet withsmall anisotropy of elongation after fracture can be suitably utilizedas a material for an application that requires high ductility at roomtemperature.

(4) In one embodiment of the magnesium alloy sheet,

Mg₁₇Al₁₂ is dispersed on a grain boundary of the magnesium-based alloy,and

the Mg₁₇Al₁₂ has an average grain size in the range of 10 nm or more and30 μm or less.

In this embodiment, very fine Mg₁₇Al₁₂ can more easily prevent fracturecaused by coarse grains and imparts higher ductility. In the embodiment,dispersed fine Mg₁₇Al₁₂ can improve corrosion resistance.

(5) In one embodiment of the magnesium alloy sheet,

an intermetallic compound containing Sr is dispersed on a grain boundaryof the magnesium-based alloy.

In this embodiment, a Sr-based compound, such as Al₄Sr, dispersed ongrain boundaries can more easily prevent fracture caused by coarsegrains and imparts higher ductility than a localized coarse Sr-basedcompound. In the embodiment, a Sr-based compound, such as Al₄Sr, canprevent coarsening of Mg₁₇Al₁₂ and imparts high ductility also in thisregard.

(6) In one embodiment of the magnesium alloy sheet,

the magnesium alloy sheet has a thickness in the range of 0.5 mm or moreand 5 mm or less.

In this embodiment, the magnesium alloy sheet with a small thickness inthis range can be suitably utilized as a material for an applicationthat requires both high ductility and high corrosion resistance as wellas a decrease in size and a decrease in thickness. In the embodiment,the magnesium alloy sheet with a large thickness in this range can besuitably utilized as a material for an application that requires highductility and corrosion resistance as well as high strength andrigidity. Although bending requires higher elongation on the outsidethan on the inside, due to its high ductility, as described above, theembodiment even with a large thickness can be easily subjected todeformation, such as bending.

Details of Embodiments of Present Invention

A magnesium alloy sheet according to an embodiment of the presentinvention is more specifically described below. Unless otherwisespecified, the element content of an alloy composition is based on themass percentage (% by mass).

[Magnesium Alloy Sheet] (Outline)

A magnesium alloy sheet according to an embodiment is formed of amagnesium-based alloy with a particular composition containing Al, Sr,Zn, and Mn in particular ranges. The magnesium-based alloy contains Al:5.0% or more and 6.5% or less, Sr: 0.2% or more and 1.0% or less, Zn:0.1% or more and 0.75% or less, and Mn: 0.1% or more and 0.5% or less,the remainder being magnesium and incidental impurities. The followingis a detailed description.

(Composition)

Al contributes mainly to corrosion resistance.

At an Al content of 5.0% or more, an intermetallic compound containingMg and Al, typically Mg₁₇Al₁₂, is formed on grain boundaries and ensurescorrosion resistance. A higher Al content tends to result in theformation of Mg₁₇Al₁₂ and higher corrosion resistance. An Al content of5.05% or more or 5.1% or more tends to result in higher corrosionresistance.

At an Al content of 6.5% or less, coarse grains composed of a compoundcontaining Al, such as Mg₁₇Al₁₂, are difficult to form, and fracturecaused by coarse grains composed of the compound is suppressed. Thisresults in high ductility. An Al content of 6.45% or less, 6.4% or less,particularly less than 6.2%, tends to result in higher ductility.

Sr contributes mainly to finer grains of a magnesium-based alloy andfiner Mg₁₇Al₁₂. Consequently, Sr contributes to improved ductility.

At an Al content of 5.0% or more as described above, coarse grains, forexample, composed of Mg₁₇Al₁₂ are easily formed on grain boundaries ofthe magnesium-based alloy, and the coarse grains tend to cause castingdefects around the coarse grains. The coarse grains and casting defectsreduce ductility. At an Al content in the above range and at a Srcontent of 0.2% or more, a Sr-based compound containing Sr, particularlya compound containing Al and Sr, such as Al₄Sr, can be preferentiallyformed over Mg₁₇Al₁₂ in the production process. This is because Al₄Srhas a higher melting point than Mg₁₇Al₁₂. Typically, a Sr-basedcompound, such as Al₄Sr, has fine grains, which act as crystal nuclei.Thus, the magnesium-based alloy tends to have fine grains. A Sr-basedcompound containing Al, such as Al₄Sr, suppresses the subsequent growth(coarsening) of Mg₁₇Al₁₂. Consequently, Mg₁₇Al₁₂ tends to have finegrains. Thus, the magnesium-based alloy can have a microstructurecontaining fine grains and containing Mg₁₇Al₁₂ and a Sr-based compound,such as Al₄Sr, finely dispersed on grain boundaries. In other words, themagnesium-based alloy can have a microstructure in which Mg₁₇Al₁₂ and aSr-based compound are finely and almost uniformly dispersed throughoutthe microstructure. Such a fine microstructure improves ductility. Ahigher Sr content tends to result in the formation of a Sr-basedcompound, such as Al₄Sr. The Sr content can be 0.25% or more or 0.3% ormore.

A Sr content of 1.0% or less results in the prevention of coarsening ofa Sr-based compound, such as Al₄Sr, and results in high ductility. At aSr content of 0.95% or less or 0.9% or less, the coarsening can be moreeasily prevented.

Zn contributes mainly to improved corrosion resistance.

At a Zn content of 0.1% or more, the formation of Mg₁₇Al₁₂, which iseffective for corrosion resistance, is promoted. Thus, even after thepreferential formation, Mg₁₇Al₁₂ is appropriately formed and improvescorrosion resistance. At a Zn content of 0.15% or more or 0.2% or more,the promoting effect is more easily achieved.

A Zn content of 0.75% or less results in promoted formation of Mg₁₇Al₁₂,prevention of coarsening, and high ductility. A Zn content of 0.74% orless or 0.73% or less results in more reliable prevention of thecoarsening of Mg₁₇Al₁₂ and high ductility.

Mn contributes to improved corrosion resistance.

At a Mn content of 0.1% or more, a compound such as Al—Mn—Fe can beformed in the production process from Fe in the melt and Al and Mn.Thus, Mn makes phase separation of Fe, which adversely affects corrosionresistance, as the above compound and thereby decreases or removes Fe.At a Mn content of 0.15% or more or 0.2% or more, the compound can bemore reliably formed, and deterioration of corrosion resistance causedby Fe can be more easily suppressed.

At a Mn content of 0.5% or less, a compound such as Al—Mn can beprevented from being excessively formed, which results in high corrosionresistance. The compound such as Al—Mn acts as a starting point offiliform corrosion, and the formation of a large amount of the compoundlowers corrosion resistance. At a Mn content of 0.45% or less or 0.4% orless, the compound such as Al—Mn can be more easily prevented from beingexcessively formed, which results in high corrosion resistance.

(Microstructure)

A magnesium alloy sheet according to an embodiment typically has a finemicrostructure. For example, when a magnesium alloy sheet according toan embodiment is a cast material produced by a continuous castingprocess, such as a twin-roll process, a magnesium-based alloyconstituting the magnesium alloy sheet has an average grain size of 30μm or less or 20 μm or less. Alternatively, for example, when amagnesium alloy sheet according to an embodiment is a rolled materialproduced by rolling, including warm rolling, of the cast material, or isa formed material produced by plastic working, such as press forming, ofthe rolled material, a magnesium-based alloy constituting the magnesiumalloy sheet has an average grain size of 20 μm or less or 15 μm or less.The rolled material has a smaller grain size and therefore higherductility than the cast material.

The average grain size can be measured, for example, by a line method.In the line method, a cross section of a magnesium alloy sheet isobserved with an optical microscope, a measuring straight line with apredetermined length is drawn on the observed image, and the number ofgrains cut by the measuring straight line is counted. The length of themeasuring straight line divided by the number of grains is the averagegrain size. For example, the length of the measuring straight line issuch that the number of grains cut by the straight line is 50 or more.Alternatively, the average grain size can be measured by a SEM-EBSDcrystal analysis using a scanning electron microscope (SEM) and electronbackscatter diffraction (EBSD) in combination.

A magnesium alloy sheet according to an embodiment has a microstructurein which fine compound grains are dispersed on grain boundaries. Morespecifically, a magnesium alloy sheet according to an embodimentcontains Mg₁₇Al₁₂ dispersed on grain boundaries of a magnesium-basedalloy, and the Mg₁₇Al₁₂ has an average grain size in the range of 10 nmor more and 30 μm or less. A magnesium alloy sheet according to anembodiment containing such fine Mg₁₇Al₁₂ is the above rolled material.

Mg₁₇Al₁₂ improves corrosion resistance, as described above. Inparticular, very finely dispersed Mg₁₇Al₁₂ with an average grain size of10 nm or more further improves corrosion resistance.

Mg₁₇Al₁₂ with an average grain size of 30 μm or less rarely causescasting defects possibly formed around coarse Mg₁₇Al₁₂ or around coarsegrains and can easily suppress deterioration of ductility caused by thecasting defects, thus resulting in high ductility. Mg₁₇Al₁₂ with asomewhat large average grain size of 15 nm or more, 20 nm or more, 30 nmor more, 50 nm or more, 100 nm or more tends to further improvecorrosion resistance. Mg₁₇Al₁₂ with a smaller average grain size of 25μm or less, 20 μm or less, 15 μm or less, 10 μm or less tends to resultin higher ductility.

A magnesium alloy sheet according to an embodiment contains anintermetallic compound containing Sr (Sr-based compound) dispersed ongrain boundaries of the magnesium-based alloy. For example, the Sr-basedcompound is a compound containing Al and Sr, such as Al₄Sr. As describedabove, Sr in the Sr-based compound probably contributes to fineMg₁₇Al₁₂. Consequently, fine Mg₁₇Al₁₂ improves corrosion resistance andductility.

A fine Sr-based compound further improves ductility. For example, theSr-based compound has an average grain size in the range of 50 nm ormore and 5 μm or less or 500 nm (0.5 μm) or more and 2 μm or less. Amagnesium alloy sheet according to an embodiment containing such a fineSr-based compound is the above rolled material or a cast materialproduced by the above twin-roll process. The rolled material containsMg₁₇Al₁₂ and a Sr-based compound finely dispersed on grain boundaries ofthe fine microstructure as described above.

The average grain size of Mg₁₇Al₁₂ and the average grain size of theSr-based compound can be measured as described below. A cross section ofa magnesium alloy sheet is observed with SEM, and Mg₁₇Al₁₂ grains andSr-based compound grains are extracted from the observed image. The areaof each grain in the cross section is determined, and the diameter of acircle with this area is considered to be the grain size. The grainsizes of 50 or more grains are averaged to determine the average grainsize.

The grain size and state of Mg₁₇Al₁₂ and the grain size and state of theSr-based compound can typically depend on the composition or theproduction conditions of the magnesium-based alloy.

(Shape, Size)

A magnesium alloy sheet according to an embodiment can have varioussizes (thickness, width, length, etc.) and various shapes depending onits use. A magnesium alloy sheet according to an embodiment is arectangular flat sheet with a predetermined uniform thickness, width,and length. Alternatively, the flat sheet is long and is wound to form acoiled material. The flat sheet or coiled material is the rolledmaterial or cast material. When a coiled material of a rolled materialis utilized as a material for a plastic worked component, such as apress formed component, the material can be continuously supplied to aprocessing apparatus, such as a press machine, thus contributing to themass production of the plastic worked component. When a magnesium alloysheet according to an embodiment is the above formed material (plasticworked component), at least part of the magnesium alloy sheet is bent toform a three-dimensional shape. Furthermore, the magnesium alloy sheetmay have a through-hole, a groove, or a protrusion and may locally havea different thickness.

The thickness of a magnesium alloy sheet according to an embodiment canbe appropriately selected. For example, the thickness may range from 0.5mm or more and 5 mm or less. In this range, when the thickness is assmall as 2 mm or less, 1.5 mm or less, or 1.0 mm or less, the magnesiumalloy sheet is suitable as a material for small thin magnesium alloystructural components, such as housings of mobile devices. In the aboverange, when the thickness is as large as more than 2 mm or 2.5 mm ormore, the magnesium alloy sheet is suitable as a material for magnesiumalloy structural components with higher strength and rigidity, such ashousings of large devices. Even when the magnesium alloy sheet has athickness of more than 2 mm and 5 mm or less, the sheet has highductility as described above and can stretch sufficiently on the outsidewhen bent in the thickness direction. For a further decrease in weightor thickness, the thickness may be 4.5 mm or less, 4 mm or less, 3.8 mmor less, 3.5 mm or less.

(Mechanical Properties)

A magnesium alloy sheet according to an embodiment typically has a highelongation after fracture even at room temperature. In particular, whena magnesium alloy sheet according to an embodiment is the above rolledmaterial, the magnesium alloy sheet has a higher elongation afterfracture at room temperature than a magnesium alloy sheet of the abovecast material. For example, the elongation after fracture at roomtemperature is 18% or more. The term “elongation after fracture”, asused herein, refers to the value in a transverse direction of amagnesium alloy sheet. When a magnesium alloy sheet according to anembodiment is a rolled material, the elongation after fracture in thetransverse direction may be lower than the elongation after fracture inthe rolling direction. Thus, when the elongation after fracture in thetransverse direction is as high as 18% or more, the elongation afterfracture in the rolling direction is expected to be higher than or equalto the elongation after fracture in the transverse direction, and thewhole magnesium alloy sheet has a high elongation after fracture.

A higher elongation after fracture results in higher ductility andeasier deformation, such as bending, and the elongation after fractureis 18.5% or more, 19% or more, or 19.5% or more.

The rolling direction and the transverse direction of a magnesium alloysheet may be determined in a simplified manner from the shape and sizeof the magnesium alloy sheet. For example, for the above coiledmaterial, the longitudinal direction of the sheet can be considered tobe the rolling direction, and the width direction perpendicular to thelongitudinal direction of the sheet can be considered to be thetransverse direction. For the above flat sheet or three-dimensionalsheet with a roughly rectangular planar shape, the long side directioncan be considered to be the rolling direction, and the short sidedirection can be considered to be the transverse direction.

A magnesium alloy sheet according to an embodiment typically has smallanisotropy of elongation after fracture even at room temperature. Inparticular, when a magnesium alloy sheet according to an embodiment isthe above rolled material, the magnesium alloy sheet has smalleranisotropy of elongation after fracture at room temperature than amagnesium alloy sheet of the above cast material. For example, thedifference between elongation after fracture in the rolling direction ofa magnesium alloy sheet and elongation after fracture in a transversedirection (hereinafter referred to as an elongation difference) is 2% orless. A smaller elongation difference results in smaller anisotropy ofelongation after fracture, higher ductility, and easier uniformdeformation, such as bending, in any direction. Thus, the elongationdifference is preferably 1.8% or less, more preferably 1.5% or less,1.3% or less, still more preferably 1.2% or less.

The elongation after fracture can typically depend on the composition orthe production conditions of the magnesium-based alloy. For example, alow Al or Zn content in the above range tends to result in a highelongation after fracture.

(Surface Texture)

A magnesium alloy sheet according to an embodiment also has high surfacetreatability when the magnesium alloy sheet has the fine microstructureas described above and contains Mg₁₇Al₁₂ or a compound such as aSr-based compound finely dispersed on grain boundaries. For example, amagnesium alloy sheet subjected to corrosion protection treatment, suchas chemical conversion treatment or anodic oxidation treatment, tends tohave a treatment layer with a uniform thickness and has higher corrosionresistance.

[Method for Producing Magnesium Alloy Sheet]

When a magnesium alloy sheet according to an embodiment is the rolledmaterial, the magnesium alloy sheet can be produced by a methodincluding a casting step of producing a cast material by a continuouscasting process, a heat-treatment step of performing solid solutiontreatment of the cast material to produce a solid solution treatedmaterial, and a rolling step of performing rolling, including warmrolling, of the solid solution treated material to produce a rolledmaterial. When a magnesium alloy sheet according to an embodiment is thecast material, the magnesium alloy sheet can be produced through thecasting step. When a magnesium alloy sheet according to an embodiment isthe formed material, the formed material can be produced by the castingstep, the heat-treatment step, the rolling step, and a processing stepof performing plastic working of the rolled material to produce aplastic worked component. The outline of each step is described below.

(Casting Step)

A cast material is preferably produced by a continuous casting process.In particular, a cast material produced by a twin-roll process that canperform rapid solidification at a solidification rate of 50 K/s or moreis more preferred because the cast material has substantially no or veryfew internal defects, such as shrinkage cavities, pores, andsegregation, or substantially no or very few coarse oxides, has a smallaverage grain size, or contains smaller crystallized precipitates (seeFIG. 3 described later). The twin-roll process is preferred because itenables the mass production of a very long cast material with apredetermined thickness and width. A higher solidification rate canresult in fewer internal defects and inclusions and finer grains. Thesolidification rate can be 100 K/s or more, 200 K/s or more, 300 K/s ormore, or 400 K/s or more. For the other casting conditions, knownconditions may be referred to.

(Heat-Treatment Step)

Solid solution treatment of the cast material to perform solid solutionof additive elements is preferred because it facilitates precipitationof precipitates of a uniform size containing the additive elementsduring warm rolling in the next rolling step. For example, theheat-treatment conditions include a heat-treatment temperature in therange of 380° C. or more and 420° C. or less and a holding time in therange of 60 minutes or more and 600 minutes or less. For the otherheat-treatment conditions, known conditions may be referred to.

(Rolling Step)

A heat-treated material produced by the heat-treatment step can besubjected to rolling, including warm rolling, to have a densemicrostructure with substantially no or fewer casting defects, such aspores, to have a finer microstructure, or to form precipitates, such asMg₁₇Al₁₂. In particular, fine precipitates, such as Mg₁₇Al₁₂, can beformed from a heat-treated material composed of a magnesium-based alloywith a particular composition containing Al, Sr, Zn, and Mn in theparticular ranges, as described above. The warm rolling conditionsinclude a material temperature in the range of 150° C. or more and 350°C. or less and a draft in the range of 10% or more and 50% or less perpass, for example. For the other rolling conditions, known conditionsmay be referred to. High density with substantially no or very fewcasting defects, the presence of texture, or the presence of a finemicrostructure (for example, an average grain size of 15 μm or less or10 μm or less) may be an indicator of the rolled material.

The rolled material may be subjected to surface treatment, such as aleveling process, polishing processing, or chemical conversiontreatment. For the conditions for the leveling process, polishingprocessing, and surface treatment, known conditions may be referred to.

<Applications>

A magnesium alloy sheet according to an embodiment can be utilized as amaterial for various applications. In particular, when a magnesium alloysheet according to an embodiment is the rolled material, the magnesiumalloy sheet can be utilized as a material for plastic worked componentssubjected to various plastic workings, such as press forming. Examplesof the plastic worked components, such as housings, of variouselectronic and electrical devices, more specifically, components ofrelatively small, portable devices, such as cellular phones and laptopcomputers, and relatively large devices, such as TVs. Other examples ofthe plastic worked components of various transports, more specifically,exteriors, such as body panels, interiors, such as sheet panels, enginecomponents, and chassis components of automobiles, aircrafts, andrailway vehicles. Still other examples of the plastic worked componentinclude various frames and structural components.

(Main Advantageous Effects)

Being formed of a magnesium-based alloy with a particular compositioncontaining Al, Sr, Zn, and Mn in particular ranges, a magnesium alloysheet according to an embodiment has high ductility and corrosionresistance.

These advantageous effects are more specifically described below in TestExample 1.

Test Example 1

Magnesium alloy sheets were formed of magnesium-based alloys withvarious compositions and were tested for ductility and corrosionresistance.

(Preparation of Samples)

Each sample of the magnesium alloy sheets is a rolled material producedby the following casting step, heat-treatment step, and rolling step.

In the casting step, a cast material (5 mm in thickness) is producedfrom a magnesium-based alloy with the composition listed in Table 1(each element content is expressed in % by mass) by a twin-roll process.Al, Zn, Sr, and an Al—Mn alloy are added to a melt of pure magnesiumserving as the base to produce a melt of a magnesium-based alloy withthe composition listed in Table 1. The solidification rate is 50 K/s ormore. An ICP spectroscopic analysis of the composition of the resultingcast material confirmed the composition listed in Table 1 (the remainderis composed of Mg and incidental impurities).

In the heat-treatment step, the cast material is subjected to solidsolution treatment. The heat-treatment temperature is 400° C., and theholding time is 300 minutes (5 hours).

In the rolling step, the heat-treated material is subjected to warmrolling to a thickness of 1 mm to produce a rolled material. Thematerial temperature in the warm rolling is 350° C.

(Ductility)

A tensile test specimen was cut from each sample of the rolled materialand was subjected to a tensile test according to JIS Z 2241 (Metallicmaterials-Tensile testing-Method of test at room temperature, 2011) tomeasure the elongation after fracture (%) at room temperature(approximately 20° C.). Two tensile test specimens α and β are cut fromeach sample. The tensile test specimens α and β are plate-like testspecimens No. 13 according to JIS Z 2241.

The tensile test specimen α was cut such that the longitudinal directionof the specimen was parallel to the rolling direction, and the tensiledirection in the test was parallel to the rolling direction. Theelongation after fracture of the tensile test specimen α is listed inTable 1 as elongation after fracture (%) in the rolling direction.

The other tensile test specimen β was cut such that the longitudinaldirection of the specimen was perpendicular to the rolling direction,and the tensile direction in the test was perpendicular to the rollingdirection. The elongation after fracture of the tensile test specimen βis listed in Table 1 as elongation after fracture (%) in the transversedirection.

The elongation after fracture in the rolling direction minus theelongation after fracture in the transverse direction, that is, thedifference of the elongations after fracture is listed in Table 1 as theelongation difference (%).

(Corrosion Resistance)

A test specimen was cut from each sample of the rolled material and wassubjected to a salt spray test according to JIS Z 2371 (Methods of saltspray testing, 2015). The appearance of the test specimen was observed.

After a neutral sodium chloride solution was sprayed for 192 hours, theappearance of each sample was visually inspected. Substantially no whiterust over the entire surface of the sample indicates high corrosionresistance and is rated “good”. White rust on part of the sample israted “white rust”. White rust over the entire surface of the sample israted “full white rust”. Table 1 shows the evaluation results of eachsample.

TABLE 1 Appearance Elongation after fracture (%) after SampleComposition (mass %) Rolling Transverse Elongation salt spray No. Al ZnSr Mn direction direction difference (%) for 192 h 1 5.1 0.21 0.2 0.223.2 21.2 2.0 Good 2 5.5 0.22 0 2 0 22.5 21.0 1.5 Good 3 5.3 0.53 0.30.3 22.7 22.2 0.5 Good 4 5.4 0.54 0.5 0.3 22.8 22.0 0.8 Good 5 5.7 0.610.2 0.3 22.0 20.8 1.2 Good 6 5.6 0.46 0.4 0.3 22.2 21.3 0.9 Good 7 5.90.72 0.3 0.4 21.8 20.5 1.3 Good 8 6.1 0.55 0.4 0.3 21.5 21.2 0.3 Good 95.8 0.62 0.9 0.4 20.8 20.3 0.5 Good 10 5.9 0.52 0.5 0.3 21.3 20.5 0.8Good 11 6 0.63 0.7 0.5 20.5 19.8 0.7 Good 12 6.2 0.43 0.3 0.4 20.2 18.91.3 Good 13 6.4 0.64 0.5 0.3 19.5 18.1 1.4 Good 101 6.2 — — 0.2 17.211.8 5.4 Good 102 5 — — 0.2 18.2 12.5 5.7 White rust 103 6.1 0.5  — 0.217.0 11.5 5.5 Good 104 6.3 0.8  0.8 0.3 12.5 10.0 2.5 Good 105 5.5 — 0.30.5 22.3 21.9 0.4 White rust 106 1.5 0.51 0.5 0.3 22.6 22.0 0.6 Fullwhite rust 107 6.7 0.71 0.2 0.4 15.0 13.8 1.2 Good 108 6.2 0.85 0.8 0.313.5 12.8 0.7 Good 109 6.1 0.51 1.2 0.3 9.7 7.5 2.2 Good

Table 1 shows that samples No. 1 to No. 13 have a high elongation afterfracture at room temperature, a small elongation difference, highductility, and high corrosion resistance. Quantitatively, for thesamples No. 1 to No. 13, the elongation after fracture at roomtemperature (elongation after fracture in the transverse direction) is18% or more, 18.5% or more in most samples, 19% or more, or 20% or morein many samples. The samples No. 1 to No. 13 have an elongationdifference of 2% or less, 1.5% or less in most samples, or 1.0% or lessin many samples. Thus, the samples No. 1 to No. 13 have small anisotropyof elongation after fracture at room temperature.

The samples No. 1 to No. 13 have a fine microstructure and have a finelydispersed grains consisting of Mg₁₇Al₁₂ or a compound such as a Sr-basedcompound on grain boundaries. FIG. 1 shows SEM images of a cross sectionof the rolled material of the sample No. 10 and a rolled material of asample No. 101 observed with SEM. The upper is the sample No. 10, andthe lower is the sample No. 101. FIG. 2 is a SEM image of the sample No.10 and is an enlarged photograph observed under magnification. In theupper in FIG. 1, FIG. 2, and the upper in FIG. 3 described later, whitegrains are formed of a Sr-based compound, such as Al₄Sr. In the lower inFIG. 1 and in the lower in FIG. 3 described later, white grains areformed of a compound composed of Al and Mn, such as Al—Mn. In FIG. 2,very fine light gray grains are formed of Mg₁₇Al₁₂. In FIGS. 1 and 2,some grains are indicated by black arrows.

The upper in FIG. 1 shows that the sample No. 10 contains a Sr-basedcompound with a grain size of approximately 1 μm or less finelydispersed on grain boundaries. FIG. 2 shows that the sample No. 10contains very fine Mg₁₇Al₁₂ with a grain size of 0.1 μm (100 nm) or lessor 50 nm or less dispersed on grain boundaries. When measured by theline method, the sample No. 10 has an average grain size ofapproximately 8 μm, the Sr-based compound has an average grain size ofapproximately 500 nm (0.5 μm), and Mg₁₇Al₁₂ has an average grain size ofapproximately 50 nm. The samples No. 1 to No. 9 and No. 11 to No. 13probably have a similar fine microstructure.

In contrast, among samples No. 101 to No. 109, the samples No. 101, No.103, No. 104, and No. 107 to No. 109 with high corrosion resistance havea very low elongation after fracture at room temperature (in thetransverse direction), only approximately 13% or less, and have lowductility. The samples No. 105 and No. 106 with a high elongation afterfracture at room temperature (in the transverse direction) have whiterust and full white rust and have low corrosion resistance. The sampleNo. 102 has a low elongation after fracture at room temperature (in thetransverse direction) and low corrosion resistance. The lower in FIG. 1shows that the sample No. 101 contains finely dispersed Al—Mn with agrain size of approximately 1 μm or less.

The reason for these results is probably that the samples No. 1 to No.13 were composed of the magnesium-based alloy with a particularcomposition containing Al, Sr, Zn, and Mn in the particular ranges andtherefore had high ductility and corrosion resistance. The samples No.101 to No. 109 composed of the magnesium-based alloy with a compositionoutside the particular ranges are discussed below for comparison.

The sample No. 101 contains Al and Mn corresponding to an AM60 alloy,which is an American Society for Testing and Materials standard alloywith high ductility, and has a microstructure containing finelydispersed grains composed of Al—Mn, as shown in the lower in FIG. 1. Thesamples No. 1 to No. 13, which contain Al and Mn, have higher ductilitythan the sample No. 101, which contain no Sr or Zn.

The sample No. 102 has a lower Al content than the sample No. 101. Thesamples No. 1 to No. 13 have much higher ductility and higher corrosionresistance than the sample No. 102.

The sample No. 103 contains Al, Zn, and Mn but no Sr. The samples No. 1to No. 13 have higher ductility than the sample No. 103.

The sample No. 105 contains Al, Sr, and Mn but no Zn. The samples No. 1to No. 13 have higher corrosion resistance than the sample No. 105.

Thus, the comparison with the samples No. 101 to No. 103 and No. 105shows that Al, Sr, Zn, and Mn are preferably contained to achieve bothhigh ductility and high corrosion resistance.

The samples No. 1 to No. 13 have higher ductility than the samples No.104 and No. 107 to No. 109, which contain Al, Sr, Zn, and Mn. A high Zncontent of 0.8% or more by mass in the samples No. 104 and No. 108, ahigh Al content of 6.7% by mass in the sample No. 107, and a high Srcontent of 1.2% by mass in the sample No. 109 tend to result in theformation of coarse compound grains and low ductility.

The samples No. 1 to No. 13 have higher corrosion resistance than thesample No. 106, which contains Al, Sr, Zn, and Mn. The sample No. 106has low corrosion resistance probably due to a low Al content of 4.5% bymass.

Thus, the comparison with the samples No. 104 and No. 106 to No. 109shows that Al: more than 4.5% by mass and less than 6.7% by mass, Sr:more than 0 and less than 1.2% by mass, Zn: more than 0 and less than0.8% by mass, and Mn: 0.2% or more by mass and 0.5% or less by mass arepreferred to achieve both high ductility and high corrosion resistance.

One reason for the fine microstructure of the rolled material of thesamples No. 1 to No. 13, for fine Mg₁₇Al₁₂, and for a fine Sr-basedcompound is probably that the cast material produced by the twin-rollprocess was used as a material. FIG. 3 shows SEM images of a crosssection of the cast material of the sample No. 10 and the cast materialof the sample No. 101 observed with SEM. The upper is the sample No. 10,and the lower is the sample No. 101. FIG. 4 shows photomicrographs of across section of the cast material of the sample No. 10 and the castmaterial of the sample No. 101 observed with an optical microscope. Theupper is the sample No. 10, and the lower is the sample No. 101.

FIG. 3 shows that the cast materials of the samples No. 10 and No. 101contain Mg₁₇Al₁₂ on grain boundaries. Although Mg₁₇Al₁₂ in the sampleNo. 10 is composed of long grains approximately 5 μm or less in length,Mg₁₇Al₁₂ in the sample No. 101 forms a continuous network on the grainboundary. For example, Mg₁₇Al₁₂ in the sample No. 101 extendscontinuously from left to right. Probably due to such Mg₁₇Al₁₂ in thecast material, Mg₁₇Al₁₂ in the rolled material of the sample No. 101tends to become relatively coarse, and the rolled material of the sampleNo. 101 had lower ductility than the rolled material of the sample No.10.

FIG. 4 shows that the cast materials of the samples No. 10 and No. 101have a microstructure. Although the sample No. 10 has an average grainsize of approximately 20 μm and has an almost uniform size, the sampleNo. 101 has a large average grain size of approximately 50 μm andcontains coarse grains in the range of approximately 60 μm or more and100 μm or less. Thus, the rolled material of the sample No. 101 rarelyhas finer grains than the rolled material of the sample No. 10 and alsoin this respect had lower ductility.

These tests show that a magnesium alloy sheet formed of amagnesium-based alloy with a particular composition containing Al, Sr,Zn, and Mn in particular ranges can have both high ductility and highcorrosion resistance.

The present invention is defined by the appended claims rather than bythese embodiments. All modifications that fall within the scope of theclaims and the equivalents thereof are intended to be embraced by theclaims.

For example, the composition and thickness of the magnesium-based alloyin Test Example 1 can be appropriately modified.

1. A magnesium alloy sheet formed of a magnesium-based alloy, themagnesium-based alloy comprising: on a mass percent basis, Al: 5.0% ormore and 6.5% or less, Sr: 0.2% or more and 1.0% or less, Zn: 0.1% ormore and 0.75% or less, and Mn: 0.1% or more and 0.5% or less, theremainder being magnesium and incidental impurities.
 2. The magnesiumalloy sheet according to claim 1, wherein the difference betweenelongation after fracture in a rolling direction and elongation afterfracture in a transverse direction in the magnesium alloy sheet is 2% orless.
 3. The magnesium alloy sheet according to claim 1, wherein themagnesium alloy sheet has an elongation after fracture of 18% or more.4. The magnesium alloy sheet according to claim 1, wherein Mg₁₇Al₁₂ isdispersed on a grain boundary of the magnesium-based alloy, and theMg₁₇Al₁₂ has an average grain size in the range of 10 nm or more and 30μm or less.
 5. The magnesium alloy sheet according to claim 1, whereinan intermetallic compound containing Sr is dispersed on a grain boundaryof the magnesium-based alloy.
 6. The magnesium alloy sheet according toclaim 1, wherein the magnesium alloy sheet has a thickness in the rangeof 0.5 mm or more and 5 mm or less.